Thrust reversers are used in aircraft to provide additional deceleration to the aircraft, for example on landing. A thrust reverser is generally, during flight, stowed or not active. When deployed, the thrust reverser acts to redirect the backwards thrust of the aircraft engine to a forward direction to slow the aircraft.
Generally a thrust reverser includes cowls that, in the stowed position, lie flat. Actuators, such as hydraulic actuators, cause the cowls to lift, on deployment, to cause redirection or reversal of the thrust. A controller, which again is commonly a hydraulic control, controls operation of the actuators.
Thrust reversers should generally not be deployed in flight. Erroneous deployment can be catastrophic and safety systems are required to prevent this. In particular, thrust reversers are provided with locks to secure the thrust reverser in the stowed position.
Commonly, thrust reversers are provided with hydraulic lock systems which are mounted on or in relation to the thrust reverser actuators. Such lock systems generally include an actuator means having a moveable element, e.g. a piston or bellows, responsive to hydraulic pressure to put the lock in the locked or unlocked position. The lock is, for increased safety, default biased to the locked position by means of, e.g., a biasing spring.
The lock is provided with a lock valve, operation of which is controlled by the thrust reverser control circuit. When the control circuit receives a command to deploy or stow the thrust reverser, it will also provide a command to the lock valve to unlock or lock the lock. The lock valve responds to control signals by selectively increasing or decreasing the pressure of hydraulic fluid supplied to the lock actuator to cause the moveable element to move to the locked or unlocked position. Lock systems that operate in this way are commonly known as Track locks. There are also mechanical locks within each of the actuators, which are actuated by the pressure in the piston head volume, and are biased closed by springs as with the Track lock. Track locks are generally referred to as secondary or tertiary locks, because they provide an additional line of defence for the system to account for possible failures within the actuator (primary) locks.
Hydraulic thrust reverser actuation systems typically include a pressure switch for monitoring the pressure state within the system which, in flight, should be isolated from supply pressure at all times to avoid erroneous unlocking of thrust reverser locks.
It is important that pressure switches only respond to ‘true’ pressure changes. Thrust reverser control systems can (and indeed must) continue to operate properly in the event of momentary transients in return pressure of the system. Such transients occur often in normal operation of an aircraft and switches should not be so sensitive that they are triggered each time such a transient occurs. The switches must, however, be responsive to real pressure changes for correct control of aircraft systems.
To avoid erroneously responding to acceptable pressure transients or fluctuations, absolute pressure switches are typically set to have a high response threshold. This gives rise, however, to the problem that relatively low control pressures are not detected.
Other systems use differential pressure sensors to avoid providing an indication in response to momentary transients in system return pressure. Transients within the system are common to the system and do not, therefore, create differentials in pressure, and differential pressure systems are only responsive to pressure differentials and not, therefore, sensitive to transients. A problem with differential pressure switches, however, is that they only detect differentials and would not, therefore, respond to a high absolute pressure throughout the system. Such pressure could be caused, for example, by a return line blockage. This would be undetectable by differential pressure sensors and could be catastrophic if not detected in flight.
FIG. 1 is a simple sectional view of a conventional pressure switch used in a thrust reverser actuation system of an aircraft.
The pressure switch comprises a hydraulic spool 1 that responds to control line pressure to drive a spring 2 loaded target 3 into range of a pair of proximity switches 4. The proximity switches respond to proximity of the target as it is moved by a sufficiently high control pressure against the force of the spring, to provide a pressure indication.
The conventional switch is arranged to respond to pressure in the control line 5 which is connected to the top of the spool. High pressure in this line could cause unlocking of the thrust reverser locks which, if undesired, e.g. during flight, could be catastrophic.
The lower end of the spool where it pushes onto the spring loaded target is dry, and is open to atmospheric pressure. A seal on the spool prevents leakage overboard at this interface. The central portion of the spool is connected to the system return line 6.
The return pressure connection into the centre spool section does not generate any force on the spool. The only purpose of this connection is to make sure that regardless of the control pressure, the external seal on the spool is only subjected to return pressure differential, minimising its friction and hence improving the switching accuracy of the switch. The conventional switch indicates pressure purely on the basis of whether the control pressure exceeds that required to overcome the spring.
A problem with this operation is that the response of the switch (based e.g. on the spring force) has to be set quite high to avoid undesired response to “normal” pressure transients, as mentioned above. The switch is set not to provide any indication in response to pressures lower than the maximum for “normal” system return transients. As mentioned above, this means that lower pressures, even if not transient pressures, would not be indicated.
The aim of the present invention, therefore, is to provide a pressure switch that is responsive to relatively small differentials between control line and return line pressures as well as to relatively high absolute pressures common to both lines, but avoiding responding to normal pressure transients.